a) Field of the Invention
The present invention relates to a compact zoom lens system which has a high vari-focal ratio and a wide field angle.
b) Description of the Prior Art
To obtain a zoom lens system which is compact and has a high vari-focal ratio, it is effective to compose a lens system, in order from the object side, of a first positive lens unit, a second negative lens unit, a third positive lens unit and a fourth positive lens unit and configure the lens system so as to change a magnification by moving each lens unit.
As conventional examples of a zoom lens system which has such a composition, there are known lens system disclosed by first through sixth embodiments of Japanese Patent Kokai Publication No. Sho 57-168209, first through fourth embodiments of Japanese Patent Kokai Publication No. Sho 63-66522, first through eleventh embodiments of Japanese Patent Kokai Publication No. Sho 63-294506, first through third embodiments of Japanese Patent Kokai Publication No. Hei 1-229217, first through third embodiments of Japanese Patent Kokai Publication No. Hei 2-153311, first through fourth embodiments of Japanese Patent Kokai Publication No. Hei 9-101459, first embodiment of Japanese Patent Kokai Publication No. 9-101458, second and fourth embodiments of Japanese Patent Kokai Publication No. Hei 6-75167, first, second and fourth embodiments of Japanese Patent Kokai Publication No. Hei 8-220439, and first and second embodiments of Japanese Patent Kokai Publication No. Hei 8-248319.
The zoom lens systems mentioned above have vari-focal ratios which are not so high though each of the zoom lens system uses consists of a small number of lens elements on the order of 12 to 13.
Furthermore, each of zoom lens systems disclosed by third through sixth embodiments of Japanese Patent Kokai Publication No. Hei 8-248319 which are other conventional examples consists of lens elements in a number on the order of 12 to 13 and has a high vari-focal ration exceeding 4.5, but allows spherical aberration and astigmatism to be varied remarkably by changing a magnification from a wide position to a tele position.
An object of the present invention is to provide a zoom lens system comprising, in order from an object side: a first lens unit having positive refractive power; a second lens unit having negative refractive power; and a third lens unit having positive refractive power; and a fourth lens unit having positive refractive power, wherein a magnification is changed from a wide position to a tele position by moving the first lens unit toward the object side, moving the second lens unit so as to widen an airspace between the first lens unit and the second lens unit, moving the third lens unit so as to reduce an airspace between the second lens unit and the third lens unit and moving the fourth lens unit so as to reserve an airspace between the third lens unit and the fourth lens unit at the tele position which is narrower than that at the wide position, wherein an aperture stop is disposed on an image side of the second lens unit and on the object side of the fourth lens unit, wherein the fourth lens unit comprises a front subunit having positive refractive power and a rear subunit having negative refractive power, and wherein the zoom lens system satisfies the following conditions (1), (2) and (3):
4.1 less than f1/|f2| less than 5.0xe2x80x83xe2x80x83(1)
0.07 less than |f2|/fT less than 0.16xe2x80x83xe2x80x83(2)
0.1 less than D4/h less than 0.7xe2x80x83xe2x80x83(3)
Another object of the present invention is to provide a zoom lens system comprising, in order from an object side: a first lens unit having positive refractive power; a second lens unit having negative refractive power; a third lens unit having positive refractive power; and a fourth lens unit having positive refractive power, wherein a magnification is changed from a wide position to a tele position by moving the first lens unit toward the object side, moving the second lens unit so as to widen an airspace between the first lens unit and the second lens unit, moving the third lens unit so as to reduce an airspace between the second lens unit and the third lens unit and moving the fourth lens unit so as to reserve an airspace between the third lens unit and the fourth lens unit which is narrower at the tele position that at the wide position, wherein the first lens unit consists, in order from the object side, of a negative lens element, a positive lens element and a positive meniscus lens element having a convex surface on the object side, wherein the second lens unit consists, in order from the object side, of a negative lens component, a negative lens component, a positive lens component and a negative lens component which are disposed with an airspace reserved between each pair of these lens components, wherein the third lens unit consists, in order from the object side, of a subunit having positive refractive power and a subunit having negative refractive power disposed with an airspace reserve between these subunits, wherein the fourth lens unit comprises a front subunit having positive refractive power and a rear subunit having negative refractive power, and wherein the zoom lens system satisfies the following conditions (1), (3) and (4):
4.1 less than f1/|f2| less than 5.0xe2x80x83xe2x80x83(1)
0.1 less than D4/h less than 0.7xe2x80x83xe2x80x83(3)
0.2 less than f3/f4 less than 1.1xe2x80x83xe2x80x83(4)